In recent years, a surface acoustic wave device (hereinafter to be referred to as a SAW device) has been used in many communications fields, and has been playing the role in reducing the sizes of portable telephones and the like because of its excellent characteristics in high frequency, compactness and easiness in mass production.
FIG. 5(a) is a top plan view for showing one example of an electrode pattern of a conventional primary-third order longitudinally coupled double mode SAW filter (hereinafter to be referred to as a double mode SAW filter). On a main surface of a piezoelectric substrate 11, there are provided three normal IDT 12, IDT 13 and IDT 14 closely disposed to each other along a propagation direction of a surface wave, with reflectors 15a and 15b disposed on both sides of these IDT's.
Each of the IDT 12, IDT 13 and IDT 14 is structured by a pair of comb electrodes having a plurality of electrode fingers, with each electrode finger inserted into a space of the other comb electrode. One of the comb electrodes of the IDT 12 is connected with an input terminal and the other comb electrode is grounded. One of the comb electrodes of the IDT 13 and one of the comb electrodes of the IDT 14 are mutually connected with each other and are connected to an output terminal. The other comb electrode of the IDT 13 and the other comb electrode of the IDT 14 are mutually connected with each other and are grounded.
The double mode SAW filter shown in FIG. 5(a) operates as follows as known. A plurality of surface waves excited by the IDT's 12, 13 and 14 are trapped between the reflectors 15a and 15b, and an acoustic coupling is generated among the IDT's 12, 13 and 14. As a result, two longitudinally coupled resonance modes of primary and third-order are excited strongly, and the filter works as a double mode SAW filter utilizing these two modes. It is known that a passing band of the double mode SAW filter is proportional to a frequency difference between the primary-order resonance mode and the third-order resonance mode.
Having a plurality of the double mode SAW filters disposed on the piezoelectric substrate and having these filters connected in cascade to improve a shape factor and guaranteed attenuation of the filter, is also a means well-known in the art.
FIG. 5(b) shows an example of frequency characteristics obtained as a result of simulating a double mode SAW filter, taking the IDT electrode pattern shown in FIG. 5(a) as an example, by using 360.degree. Y-cut X-propagation LiTaO.sub.3 as a piezoelectric substrate, using 18 pairs of the IDT 12, using 18 pairs of the IDT 13, using 18 pairs of the IDT 14, using 500 reflectors, assuming a pitch ratio L.sub.t /L.sub.R of the reflectors to the IDT's as 0.990, assuming a center frequency of 1.5 GHz, and assuming a necessary pass band width as 24 MHz.
However, when an attempt is made to realize a wide-band double mode SAW filter by using the conventional normal IDT electrode pattern, the following trend is observed as is clear from the filter characteristic shown in FIG. 5(b). The shape factor at a high-pass side standardized by a center frequency is not as good as the shape factor at a low-pass side standardized by a center frequency, and the attenuation level is decreased by 13 dB at around 1.54 GHz and then increases. (A small ripple near the pass band shown in FIG. 5(b) is attributable to the reflectors 15a and 15b, and this becomes smaller after being converted into a bulk wave in the actual product. Therefore, this has no practical problem.
In order to overcome the above problem, a plurality of double mode SAW filters having similar characteristics are connected in cascade, whereby to improve the characteristics. However, it is not possible to eliminate the above-described attenuation level decreasing characteristic itself by this method, and there has been a problem that an insertion loss increases two times or three times by the known cascade connection.
Further, there has recently been a demand for narrowing an interval between channels from the needs of effective utilization of frequencies. There has also been a demand for low loss and high attenuation in the filters used. However, these demands cannot be met by the conventional double mode SAW filters that have what is called the attenuation level decreasing characteristic at the high-pass side in the vicinity of the passing band.
In order to solve the above problems, there is also proposed a ladder type SAW filter. However, this also has a problem that the use of this filter is limited as it is not possible to obtain sufficient attenuation in the frequency at a distance from the center frequency of the pass band.
With a view to solving the above-described problems, it is an object of the present invention to provide a structure of IDT electrodes and an SAW using this structure that has improved both the attenuation at a high-pass side near the passing band and spurious signal response in this area.